1. Field of the Invention
The present invention generally relates to waveguide filters, and more particularly to utilizing electric (E) field or TM modes in evanescent waveguides.
2. Brief Discussion of the Related Art
Radio transmitters and receivers require filters to remove or suppress unwanted frequencies from being transmitting or received. The transmitter portion of a radio may generate frequencies that will interfere with the radio system, or that may be prohibited by a radio frequency spectrum governing body. The receiver may need to suppress unwanted signals at different frequencies generated by the transmitter, or received from an external source, which would adversely affect the performance of the receiver.
At millimeter-wave frequencies sources of unwanted frequencies include the local oscillator frequency, image frequencies from the mixer, and the transmitter frequencies (in the case of the receiver). The frequencies generated by the mixer and the local oscillator are functions of the selected radio architecture. The closer the oscillator frequency (or its harmonics) is to the transmitter frequencies, the more difficult it is to remove the undesired frequency. However, wider spaced frequencies may result in more complex circuitry resulting in a more expensive radio implementation. A small separation between the transmit and receive frequencies can result in unwanted high power transmit frequencies leaking into the receiver. The separation between the transmit and receive frequencies is usually specified by the licensing bodies and the system operators. The radio designer may not have control over this specification.
To suppress unwanted frequencies below an acceptable power level, a filter element is required in the signal path. The filter element discriminates between the desired and undesired frequencies based on the wavelengths of the signals. At millimeter-wave frequencies the difference between the wavelengths is very small, resulting in very high manufacturing tolerances.